The technology, called ultrasonic cross-device tracking, embeds high-frequency tones.
These ultrasound “beacons” emit their audio sequences with speakers, and almost any device microphone—like those accessed by an app on a smartphone or tablet—can detect the signal and start to put together a picture of what ads you’ve seen, what sites you’ve perused, and even where you’ve been
For More Info:
http://arstechnica.com/security/2016/11/how-to-block-the-ultrasonic-signals-you-didnt-know-were-tracking-you/
